Many cast parts require the formation of voids therein. An example of such castings includes impellers of pump stages in centrifugal submersible pumps.
One method for forming such castings is the lost foam method. In the lost foam method, consumable patterns, such as polystyrene foam plastic, are coated with a permeable refractory wash and then embedded in foundry sand to be replaced by molten metal. The molten metal vaporizes the foam-type pattern and a metal duplicate of the pattern is the result. The lost foam method is advantageous because it allows for net shape capacity, i.e., the ability to produce a finished or near-finished casting that requires little or no machining. However, the lost foam method is unsuitable for the geometric particularities of the hydraulic passages required in complex castings, such as impellers and diffusers. The lost foam process is not suitable to form some complex castings because of process shortcomings including limitations in thickness of the walls as well as major costs associated with tooling, low consistency in the molding stage, low surface finish and low compression strength. Additionally, particular types of geometries result in warping of the castings. Therefore, such castings do not achieve a near net shape.
Another method for forming castings is sand molding. Sand molding uses high strength chemically bonded sand. Sand molding is not a preferred choice to form net shape parts because of mold wall movement. Additionally, some specific requirements of sand molding may only be met by high quality foundries.
Therefore, it is desirable to develop a method that results in near net shape, that is low in cost, and that avoids the shortcomings of the lost foam process. One solution is proposed in U.S. Pat. No. 4,691,754, which teaches molding a destructible layer around a first core by placing the first core into a molding machine. Partially pre-expanded polystyrene pellets are applied to the core and fully expanded, via a steam expansion step. However, a drawback with this method is that the core is exposed to moisture during the steam expansion step. It is desirable to avoid exposing the core, which is typically hygroscopic, to moisture. If the core is exposed to moisture, molten metal will come in contact with the moisture absorbed by the core when the molten metal is poured into the core. The exposure of molten metal to moisture is undesirable.